You would then become sidetracked and further learn that approximately four rodent hairs are allowed in a jar of peanut butter, that an average of 60 thrips are allowed in 100 grams of frozen broccoli, that 10 grams of hops are allowed to contain 2,500 aphids and that 5 milligrams of rat excreta in a pound of sesame seeds is A-OK with the FDA.

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What you would not learn is why the FDA might put a limit on insects' heads and not other parts of their anatomy, what rat excreta tastes like and what sort of person takes a job that entails searching for insect heads in fig cookie innards. To find these things out, you would have to pay a visit to one of the FDA's regional filth labs. You would, but now you don't have to because I'm doing it for you.

I have arranged to meet with an entomologist named Dana Ludwig, who works in the FDA's Alameda, Calif., Filth Lab, which analyzes thousands of samples of foods, most imported from the Pacific Rim, each year. In a moment of social ineptitude, I have asked Ludwig if the ludwig is a relative of the earwig. Straight off the bat, I have my foot in my mouth. I should be used to having feet in my mouth, for humans are eating insect parts all the time without knowing it.

According to an Ohio State University Extension fact sheet, most Americans unintentionally swallow 1 to 2 pounds of insects and insect pieces each year. Insects are very lightweight. If you think about how many of them it would take to make 2 pounds (and I advise you not to), you will begin to appreciate the somewhat shocking dimensions of our entomophagous intake.

The Alameda Filth Lab employs several analytical entomologists. Ludwig refers to them collectively as "filth people." Just inside the lab doorway, we stop to go over our clothing with a lint roller. A sign on the wall says, "Pet-Hair Free Zone." Ludwig is looking at my shirt. The look says that there's a name for me too, somewhere in the neighborhood of "filth person."

"There's a lot of hair on your shirt," says Ludwig as nicely as she can. The problem turns out to be my pet angora sweater. Ludwig covers me up with a lab coat. If my sweater were to shed into a food sample, some hapless Third World manufacturer might be cited for an infestation of lavender angora rabbits.

For demonstration purposes, Ludwig has set aside a bag of imported black bean wafers. Earlier in the morning, she measured out a sample of the wafers and put them in a beaker with boiling hydrochloric acid. Two hours later, the acid has digested the black bean wafer ingredients, leaving nothing solid behind but "the filth." Ludwig sieves the liquid to isolate the filth, which looks but probably does not taste like a teaspoon of melted coffee ice cream. She then scrapes it onto a "filth plate," which she slides under her microscope.

Ludwig shows me the head of a book louse, a mite fragment, a confused flour beetle underwing fragment, assorted hairs and an ant head. The magnified ant head is beautiful, a fragment of translucent amber, like what's left on your tongue in the morning when you fall asleep with a Ricola in your mouth. I ask Ludwig why there are so many more heads than bodies. I am trying to imagine the scenario that would result in an ant's head winding up in the flour sack while the rest of its body continues along its merry way. I am one confused flour beetle.

Ludwig explains that insects' "head capsules" are often more durable than their bodies. "This is especially common with larvae and caterpillars, where the body parts are soft and really get messed up" in the milling process. In other words, the bodies are in the food too, they're just not countable.

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What do these insects that we are eating every day taste like? FDA entomologist Steve Anghold told me that if you have enough aphids ground up in a batch of hops, it might conceivably make the beer taste sweeter, because aphids secrete a sweet fluid. In fact, he went on to say, ants "herd aphids like cattle and milk 'em," feeding the sweet fluid to their ant infants. "That's why aphids are called ant cows," he said. It was one of those unsettling journalistic moments where you wonder whether your source has been having an especially dull afternoon and is having you on for the fun of it.

I ask Ludwig if a couple dozen beetle larvae would change the taste of a food. She says the insects she typically deals with wouldn't impart much flavor, but that "their metabolic byproducts probably don't taste very good." I ask her what exactly she means by "metabolic byproducts." She says, "Their waste materials." She isn't talking about coffee grounds and recyclables. Not only do you have to put up with thrips in your broccoli, you have to put up with thrip excreta.

If it makes you feel any better, none of this filth is bad for you. With the exception of the dermested beetle larvae, which have hook-shaped hairs that become embedded in your intestines and prompt all manner of gastroenterological sturm und drang, the insects encompassed in the FDA's Food Defect Action Levels are objectionable either on a purely aesthetic level, or as an indicator of unsanitary warehouse conditions.

On the contrary, meals made from "microlivestock," as edible insects are called by those who enjoy eating them, are good for you. According to the Ohio State fact sheet, caterpillars have as much protein as beef, a fraction of the fat, 10 times the iron and way more riboflavin and thiamine. Plus the ranches take up much less room and can be staffed by cowboy ants hired away from low-paying aphid-herding jobs.

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Ludwig's area of expertise is filth hair identification. On her desk between the copy of "World of Moths" and an 8-by-10 color photograph of Colorado potato beetles mating, is a diploma in hair and fiber microscopy. The more common filth hairs -- rats, dogs, cats, mice -- she can identify under the microscope by sight. For less common specimens she consults highly esoteric reference books and/or a cabinet of "authentics": sample animal hairs culled from zoos.

She opens a drawer and shows me a glass slide with a mongoose hair fixed to it, and another from a ring-tailed cat. While Ludwig is off attending to a sample of chili paste, I pull open another drawer. This one contains human hairs of various ethnicities. "Japanese arm hair," says one label. There are Chinese hairs, Caucasian hairs, Filipino hairs, knuckle hairs, eyelashes, eyebrow hairs. Without saying a word, Ludwig reaches in front of me and slides the drawer shut, leading me to wonder whether somewhere in that collection is an authentic human pubic hair.

Ludwig and her colleagues also make use of excreta "authentics," glass vials of teeny tiny sample turds. I notice one labeled "caterpillar excreta." Each unit in the vial is as small as a cake crumb.

One more reason to ranch caterpillars and not cows.

Mary Roach

Former Salon columnist Mary Roach is the author most recently of "Gulp: Adventures on the Alimentary Canal." Her previous books include "Stiff," "Spook" and "Packing for Mars."